EE-403 Optical Source

7/28/2019 EE-403 Optical Source

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Optical Fiber

Communicat ion

OpticalSources

Engr.Mujeeb [email protected]

Lecturer ( Dep of Electrical

Engineering)



©irfan khan

OPTICAL SOURCES



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Optical Sources

There are two types of light sourcesused in fiber optic:

Light Emitting diode (LED)

LASER diodes (LD’s)

LED’s may have two types of source:

Surface-emitting LED

Edge-emitting LED



• A light-emitting diode (LED) is a semiconductor light source.

• LEDs are used as indicator lamps in many devices and are

increasingly used for other lighting.

• Early LEDs emitted low-intensity red light, but modern versions are

available across the visible.

• The LED consists of semiconducting material doped with impuritiesto create a p-n junction.

• As in other diodes, current flows easily from the p-side or anode, to

the n-side or cathode, but not in the reverse direction.

• Charge-carriers—electrons and holes—flow into the junction from

electrodes with different voltages. When an electron meets a hole, it

falls into a lower energy level, and releases energy in the form of a

photon.

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http://en.wikipedia.org/wiki/Doping_%28semiconductor%29

http://en.wikipedia.org/wiki/P-n_junction

http://en.wikipedia.org/wiki/Anode

http://en.wikipedia.org/wiki/Cathode

http://en.wikipedia.org/wiki/Electron

http://en.wikipedia.org/wiki/Electron_hole

http://en.wikipedia.org/wiki/Electrode

http://en.wikipedia.org/wiki/Energy_level

http://en.wikipedia.org/wiki/Energy

http://en.wikipedia.org/wiki/Photon

http://en.wikipedia.org/wiki/Photon

http://en.wikipedia.org/wiki/Energy

http://en.wikipedia.org/wiki/Energy_level

http://en.wikipedia.org/wiki/Electrode

http://en.wikipedia.org/wiki/Electron_hole

http://en.wikipedia.org/wiki/Electron

http://en.wikipedia.org/wiki/Cathode

http://en.wikipedia.org/wiki/Anode




http://en.wikipedia.org/wiki/Doping_%28semiconductor%29



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LASER diodes are: Fabry- Perot (FP-LD)

Distributed feed back (DFB- LD)

Vertical cavity surface-emitting Laser

(VCSE-LD)

All light emitters are complex

semiconductors that convert an

electrical current in to light.

Optical Sources



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The conversion process is fairly

efficient as it creates very little heat

compared to the heat generated byincandescent lights.

Optical Sources



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Small size

High radiance(Emit a lot of light in a small area)

Small emitting area(Area is comparable to the dimensionof optical fiber cores)

LED’s and Laser diodes are of interestfor fiber optic because of five inherentcharacteristics.

Optical Sources



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Very long life(offer high reliability)

Can be modulated at high speed

LED’s are used as visible indicator in

most electronics equipment.

LASER diodes are most widely used

in compact disk players.

Optical Sources



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Intentionally Left Blank



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How an atom can be stimulated ?

Atom is stimulated by using light energy.

Energy can also be imparted to an

atom by heat, Electrical force or by

collision with other particles.

How Light is Generated



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Atoms contain two specific types of

energy:

Binding energy of nucleus. Electron energy.


Electrons have different specificenergy levels in which they move

around the nucleus.



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Forbidden zone between each energy

level is known as “Energy gap”.

For an electron to move from oneenergy level to another, it must gain

or lose the exact energy difference

between the two levels.




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When the electron is orbiting in the

nucleus at its lower energy level E1 ,

it is said to be in its unexcited or ground state.

Bohr model of the hydrogen atom




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+ r E1

E2

E3 Ground state

E5E4

E3

E2

E1

Energy level diagram of the

hydrogen atom.

Bohr model of the hydrogen atom

showing permissible electron energy

levels.




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Direct Transition.

When the stimulated electron dropsdirectly back to its ground state, its

called direct transition.

Indirect Transition.

When the electron returns to itsground state in two or more steps,its called indirect transition.




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The result of direct transition is

emission of radiations of a single

wavelength.

The result of indirect transition is the

emission of radiations of two or

more wavelengths.




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Electron Energy Absorption-Direct transition

l =122nm

E1

E2

E3

E4

E5Excited state




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Direct transition

l =122nm

E1

E2

E3

E4

E5Excited state




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Electron Energy Absorption-Indirect transition.

l =103nm (UV)

E1

E2

E3

E4

E5Excited state




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Indirect transition

E1

E2

E3

E4

E5Excited state




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Semiconductor Material

• Semiconductors

– Conduction properties lie between conductor and insulator

– Conduction properties can be described byEnergy Bands

– At low temperature conduction band iscompletely empty

– By raising the temperature, electron exitedacross the band gap.



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Li ht E itti Di d (LED )



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LED is a solid-state component andhas nearly unlimited life

expectancy.

It is physically more sturdy than its

glass-encapsulated predecessors and

requires far less operating power.

Light-Emitting Diodes (LEDs)

Best Light source for bit rates less

than approx 100 – 200 Mb/s and

multimode fiber.



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LED is a PN junction diode that

emits light through the recombination

of electrons and holes when current is

forced through its junction.

LED Operation



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+-

Recombined

electron and hole

Emitted Photon

P-Type Region

(Anode)

N-Type Region

(Cathode)

Basic operation of a light emitting diode

++

+

-

--

-

--

--

--

-

--

---

+

++

++

++

++

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

Emitted Photon

LED Operation



How does it work?

P-n junction ElectricalContacts

A typical LED needs a p-n junction

Junction is biased to produce even more

e-h and to inject electrons from n to p for recombination to happen

There are a lot of electrons and holes at

the junction due to excitations

Electrons from n need to be injected to p

to promote recombination

Recombination

produces light!!



Injection Luminescence

in LED

Under forward bias – majority carriers from both sides of the junctioncan cross the depletion region and entering the material at the other side.

Upon entering, the majority carriers become minority carriers

For example, electrons in n-type (majority carriers) enter the p-typeto become minority carriers

The minority carriers will be larger minority carrier injection

Minority carriers will diffuse and recombine with the majority carrier.

For example, the electrons as minority carriers in the p-region willrecombine with the holes. Holes are the majority carrier in the p-region.

The recombination causes light to be emitted

Such process is termed radiative recombination.



Recombination and Efficiency

eVo

Eg

p n+

h =Eg

Eg

p n+ (a) (b)

Electrons in CB

Holes in VB

EC

EV

EF

◘Ideal LED will have all injection electrons to take part in the recombination process

◘In real device not all electron will recombine with holes to radiate light

◘Sometimes recombination occurs but no light is being emitted (non-radiative)

◘Efficiency of the device therefore can be described

◘Efficiency is the rate of photon emission over the rate of supply electrons



Calculate

• If GaAs has Eg = 1.43ev• What is the wavelength, lg it emits?

• What colour corresponds to the

wavelength?



Joseph C. Palais

6.1

32

The released energy takes the form of a photon.

-19 -341 1.6 10 , 6.626 10eV J h J s

(6.1)

h f W g

g W

c h

l

c h W g

LIGHT EMITTING DIODES

Now

Wg is in joules and l is in meters



Construction of Typical LED

Substrate

n

Al

SiO2

Electrical

contacts

p

Light output



LED Construction

Efficient light emitter is also an efficient absorbers of

radiation therefore, a shallow p-n junction required.

Active materials (n and p) will be grown on a lattice

matched substrate.

The p-n junction will be forward biased with contacts

made by metallisation to the upper and lower surfaces.

Ought to leave the upper part ‘clear’ so photon can

escape.

The silica provides passivation/device isolation and

carrier confinement



Efficient LED

Need a p-n junction (preferably the samesemiconductor material only different dopants)

Recombination must occur Radiativetransmission to give out the ‘right coloured LED’

‘Right coloured LED’ hc/l = Ec-Ev = Eg so choose material with the right Eg

Direct band gap semiconductors to allow efficientrecombination

All photons created must be able to leave thesemiconductor

Little or no reabsorption of photons



CandidateMaterials

Direct band gapmaterials

e.g. GaAs not Si

UV-ED l ~0.5-400nm

Eg > 3.25eV

LED - l ~450-650nm

Eg = 3.1eV to 1.6eV IR-ED- l ~750nm- 1nm

Eg = 1.65eV

Readily doped n or p-typesMaterials with refractiveindex that could allow lightto ‘get out’



Typical Exam Question

Describe the principles of operation of an LED and state the

material’s requirements criteria to

produce an efficient LED.

(50 marks)



Visible LED

Definition:LED which could emit visible light, the band gap of the materials that we use must be in the region of visible wavelength = 390- 770nm. This coincides with the energy value of 3.18eV- 1.61eV whichcorresponds to colours as stated below:

Violet ~ 3.17eV Blue ~ 2.73eV Green ~ 2.52eV Yellow ~ 2.15eV Orange ~ 2.08eV

Red ~ 1.62eV

Colour of an LED

should emits

The band gap, Eg thatthe semiconductor must

posses to emit eachlight



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LED Driver Circuits

The optical output of LED isapproximately proportional to the

drive current.

LEDs are usually driven with either a

digital signal or an analog signal.

The key concern is driving the LED

so that maximum speed is achieved.



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LED Structures

• LED for optical used should have

– High radiance out put

– High quantum efficiency

– High confinement of the charge carrier to the

active region of PN junction

– For this double hetrostructure configuration isused



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LED Structures

Radiance or brightness:

It is a measure ,in watts, of the optical power radiated into

a unit solid angle per unit area of emitting surface

Quantum efficiency:

It is related to fraction of injected electron-hole pairs that

recombine radiatively.



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LED Structures

Charge carrier confinement:It is used to achieve a high level of radiative

recombination in the active region of the device.

Optical confinement:

It is used to prevent absorption of the emitted radiation by

the material surrounding the pn junction

*By confining the charge carrier and optical emission high

radiance and high quantum efficiency can be achieved



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LED Structures

To achieve carrier and optical confinement doubleheterostructure of two different alloy layers on each side of

active region is used.

The band gap differences of adjacent layers confine the

charge carriers, while the difference in the indices of

refraction of adjoining layers confine the optical field to the

central active region.

This dual confinement leads to both

high efficiency and high radiance.



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• Two different alloy layers on each side of

the active region

• Recombination region is maximumconfined to 0.3 um.

• Two LED configuration

– Surface emitter

– Edge emitter

LED Structures



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*Energy Bands in a Double Heterojunction

Double-heterostructure configuration



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– Emitting region is perpendicular to the axis of the fiber

– The circular active area in practical surface

emitters is normally 50 um in diameter and upto 2.5 um thick.

– The emission pattern is essentially isotropic

with a 120 half power beam width.

Surface-emitting LED

S f E itti LED’



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Surface-Emitting LED’s

The surface-emitting LED (S-LED) emits light

over a vary wide angle. This type of light source

is often called a Lambertian emitter, because of

the nature of the emission pattern

This broad emission angle is attractive for use as

an indicating LED, therefore it is difficult to

focus more than a small amount of the total light

output in the fiber.

S f E itti LED’



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Surface-Emitting LED’s

The key advantage of surface-emittingLED’s is their low cost, making these

light emitters the dominant type in use.



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High radiance surface emitting LED. The active region is limited to a

circular section having an area compatible with the fiber core end

face

Ed E itti LED’



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Edge-Emitting LED’s

This type has a much narrower angleof light emission and also has asmaller emitting area.

This allows a larger percentage totallight output to be focused into thefiber core.

The disadvantage of E-LED’s is thatthey are very temperature sensitivecompared to S-LED’s.



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Edge emitting double heterojunction LED.



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Light emission cone



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Light-emission cone

Only light falling within a cone defined by the critical angle will be

emitted from an optical source.



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Some LED and Laser Diode Material Mixtures and

their Characteristics



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Quantum Efficiency

The active region of an ideal LED emits one photon for

every electron injected.

Each charge quantum-particle (electron) produces one

light quantum-particle (photon).

Thus the ideal active region of an LED has a quantumefficiency of unity.

The internal quantum efficiency is defined

ηint =number of electrons injected into LED per second

number of photons emitted from active region per second

Quantum Efficiency



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Quantum Efficiency

The external quantum efficiency is defined as

ηext =number of electrons injected into LED per second

number of photons emitted into free space per second

The external quantum efficiency gives the ratio of the

number of useful light particles to the number of

injected charge particles.



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Coupling light output to a fibre is the most difficult and

costly part of manufacturing a real LED or laser device



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Laser Diodes



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“LASER” is an acronym of

Light Amplification by

Stimulated Emission of R adiations.

Laser Diodes

Advantages:



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g

Ideal laser light is single-wavelength only. This is not

exactly true for communication lasers.

Lasers can be modulated (controlled) very precisely.

Lasers can produce relatively high power . Indeed some

types of laser can produce kilowatts of power. In

communication applications, semiconductor lasers of

power up to about 20 milli watts are available

Because laser light is produced in parallel beams, a high

percentage (50% to 80%) can be transferred into the

fibre.

Disadvantages:



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g

Lasers have been quite expensive by comparison with

LEDs.Reason: Temperature control and output power control is

needed.

The wavelength that a laser produces is a characteristicof the material used to build it and of its physical

construction. Lasers have to be individually designed for

each wavelength they are going to use.

Amplitude modulation using an analogue signal is

difficult with most lasers because laser output signal

power is generally non-linear with input signal power

A Id l B f L R di i



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An Ideal Beam of LASER Radiation

Properties of LASER



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LASER has three main characteristics:

Monochromatic

Coherent

Collimated

p

Properties of LASER



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Radiation which occurs at a single wavelength or color.

Monochromatic

p

Properties of LASER



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It means all waves vibrate in step, there byconstructively reinforcing each adjacent

wave.

Constructive interference Coherent light

Destructive interference Incoherent light

=

=

Coherent

A.

B.

OPTICAL INTERFERENCE

p

Properties of LASER



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It means rays are travelling in the samedirection on parallel paths.

Source

Target area

A

Source

Collimating lens

B

Collimated

p

LASER Principles



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• Laser action is the result of three keyprocess

1. Photon absorption

2. Spontaneous emission

3. Stimulated emission

p



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Laser transition processes

LASER Principles



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LASER Principles



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Spontaneous emission

When an electron is elevated to a high energy state thisstate is usually unstable.

Electron will spontaneously return to a more stable state

very quickly (within a few picoseconds) emitting a photon as

it does so.

When light is emitted spontaneously:

•Its direction and phase will be random

•Wavelength will be determined by the amount of energy

that the emitting electron must give up.

LASER Principles



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Stimulated emission

In some situations when an electron enters a high energy

(excited) state it is able to stay there for a relatively long

time (a few microseconds) before it changes state

spontaneously.

When an electron is in this semi-stable (metastable) high

energy state it can be “stimulated” by the presence of a

photon of light to emit its energy in the form of another

photon.

Stimulated emission



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**The photon that triggered (stimulated) the

emission itself is not absorbed and continues along

its original path accompanied by the newly emitted

photon.

**It is of fundamental importance to understand that

when stimulated emission takes place the emitted

photon has exactly the same wavelength, phase and

direction as that of the photon which stimulated it.

Stimulated emission

LASER Principles



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• Photon absorption

E2

E1

LASER Principles



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Absorption

l =122nm

E1

E2

E3

E4

E5Excited state

LASER Principles



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–Spontaneous emission

E2

E1

LASER Principles



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Spontaneous Emission

l =122nm

E1

E2

E3

E4

E5Excited state

LASER Principles



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• Stimulated emission

E2

E1

LASER Principles



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Under normal conditions, thepopulation of unexcited atoms greatly

exceeds the population of excited atoms.

This result, predominance of the

spontaneous emission and the resultingradiation is largely incoherent.

Population Inversion



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For a LASER to operate, situationmust be reversed, called population

inversion.

Once the majority of the atoms in a

material are in an excited state,

stimulated emission will be greaterthan spontaneous.

Population inversion is achieved by various

“Pumping” techniques.

Energy Pumping



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The energy population in a Lasingmedium is inverted by a methodknown as energy pumping.

Energy can be pumped, into amaterial in a number of ways,depending upon the composition of the material

Optical pumping.

Electronic pumping.



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The amount of light leaving the end mirror (partially

reflected) equals the amount of energy being pumped into

the cavity ,minus losses .

The amount of reflectivity needed in the end

mirrors is a function of the amount of gain in the

cavity.

Types of LASER Diodes



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Fabry-Perot LASER Diode

Distributed Feedback LASER Diode

Vertical Cavity Surface-emitting LASER

Diode

Fabry-Perot LASER Diode



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– Radiation is generated by the cavity

– Cavity size is very small like 250-500 Um long, 5-15 um wide and 0.1 to 0.2 um thick

– Reflecting mirrors are used at both sides of

the cavity – Mirrors provide strong feedback to the cavity

to have the mechanism

– The side of the cavity are formed by

roughening the edges to reduce unwantedemission

Fabry-Perot resonator cavity



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Fabry-Perot resonator cavity for a laser diode. The cleaved crystal ends function as

partially reflection mirrors. The unused end ( the rear facet) can be coated with

dielectric reflector to reduce optical loss in the cavity. Note that the light beam

emerging from the laser forms a vertical ellipse, even though the lasing spot at the

active-area facet is a horizontal ellipse.



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DFB LASERs offer better performance, less

noise and also higher cost than FP.

They are nearly monochromatic.

DFB LASERs are used for high speed digitalapplication and for most anloge application because of their faster speed, low noise andsuperior linearity.

Lasing is achieved from the Braggreflectors (gratings) along the length of thediode

DFB laser



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Relationship between optical power and laser diode drive current. Below

the lasing threshold the optical output is a spontaneous LED-type

emission.

Vertical Cavity Surface-emitting LASER Diode



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VCSELs don’t use monitor photodiode as

they emit their light in a vertical plane perpendicular to the semiconductor wafer.

Dramatically reduce the cost of LASERs tonear those of LEDs

Available @ 850 nm

Generates interest @ 850 window

Vertical Cavity Surface-emitting LASER Diode



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1300 nm VCSELs to be ready

Currently Conventional LASERs

used @ 1300nm

VCSELs are manufactured in such a

way that they are ideal for

application requiring arrays of devices.



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The process of putting information onto a light wave is

called modulation.

Two types:

1. Direct Modulation

2. External Modulation

For Data rates of less than approximately 10 Gb/s

(Typically 2.5 Gb/s), the process of imposing information on

a laser-emitted light stream can be realized by direct

modulation.

For higher data rates external modulator is used to

temporally modify a steady optical power level emitted by

the laser.




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The basic limitation on the direct modulation rate of laser

diodes depends on:

1. Spontaneous emission carrier lifetimes.

2. Stimulated emission carrier lifetimes.

3. Photon lifetime.

Spontaneous carrier life time is a function of the semiconductor band

structure and the carrier concentration.

Stimulated carrier life time depends on the optical density in the lasing

cavity.

The photon lifetime is the average time that the photon resides in the

lasing cavity before being lost either by absorption or by emission through

the facets.




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Pulse modulation is carried out by modulating the

laser only in the operation region above

threshold.

In this region the carrier lifetime is now shortened

to the stimulated emission lifetime, so that the

high modulation rates are possible.

Modulation methods



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Directly modulated laser diode, the modulation

frequency can be no larger than the frequency of therelaxation oscillations of the laser field.

The relaxation oscillation depends on both the

spontaneous lifetime and the photon lifetime.

Spontaneous

life timePhoton

life timeThreshold

current

Total

current

Relaxation oscillation peak



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Directly modulated lasers can not operate

smoothly at data rates greater than 2.5 Gbps. It

is suitable to use external modulators above this

data rate.

There are two types of external modulators

1.Electrooptical modulator

2.Electroabsorption modulator

1.Electrooptical modulator



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1.Light beam is split in half

2.A high-speed electric signal then changes the

phase of the light signal in one of the paths.

3.The constructive recombination produces a

bright signal and corresponds to a 1 pulse.

4. Destructive recombination corresponds to a

0 pulse.

The electrooptical (EO) modulator typically is

made of lithium niobate (LbNiO3).

2. Electroabsorption modulator



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The electroabsorption modulator (EAM) typically is

constructed from indium phosphide (InP).

It operates by having an electric signal change the

transmission properties of the material in the light

path to make it either transparent during a 1 pulse or opaque during a 0 pulse



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Bias Current Electronic Modulation Current

Constant Optical Output Power Modulated Optical Output Power

Pcw P (t)

Operational concept of generic external modulator



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CW

Input Light

Modulated

Input Light

Phase

Shifter

Beam

Splitter

Beam

Combiner

Operational concept of an electrooptical lithium niobate

external modulator

Temperature Effects



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Threshold current Ith (T) of laser diode also depends on the

temperature. This parameter increases with temperature in

all types of semiconductor lasers.

Adjustment in the dc-bias (varies with temperature)current level is necessary for constant optical power level.

One possible method for achieving this automatically is

an optical feedback scheme.

So, output optical power can be stabilized in two ways:

1. Controlling dc-bias current level.

2. Controlling temperature level.

Temperature Effects



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The Photodetector compares the optical power with areference level and adjusts the dc-bias current level

automatically to maintain a constant peak light output relative

to the reference.

Another standard method of stabilizing the optical output of

a laser diode is to use a miniature thermoelectric cooler.

Optical feedback can be carried out by using a

photodetector either to sense the variation in optical power

emitted from the rear facet of the laser or to tap off andmonitor a small portion of the fiber-coupled power emitted

from the front facet.

Normally both methods are used together

Temperature dependence



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Temperature-dependent behavior of the optical output power

as a function of the bias current for a particular laser diode.



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Construction of a laser transmitter that uses a rear-facet photodiode for

output monitoring and a thermoelectric cooler for temperature stabilization



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High-performance optical transmitter containing a DFB laser a fiber

EE-403 Optical Source

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